Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball showing a great flight distance on driver shots. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein the spherical core has a difference between absorbance of the absorption peak appeared at 1560±30 cm −1  at a surface thereof and absorbance of the absorption peak appeared at 1560±30 cm −1  at a center thereof of 0.024 or more, when measuring the spherical core with a Fourier transform infrared spectrophotometer.

FIELD OF THE INVENTION

The present invention relates to a golf ball excellent in flyingperformance, in particular, an improvement of a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for improving a flight distance on driver shots, forexample, there are methods of using a core having high resilience andusing a core having a hardness distribution in which the hardnessincreases toward the surface of the core from the center thereof. Theformer method has an effect of enhancing an initial speed, and thelatter method has an effect of a higher launch angle and a lower spinrate. A golf ball having a higher launch angle and a low spin ratetravels a great distance.

For example, Japanese Patent Publications Nos. S61-37178 A, S61-113475A, S61-253079 A, 2008-212681 A, 2008-523952 T and 2009-119256 A disclosea technique of enhancing resilience of the core. Japanese PatentPublications Nos. S61-37178 A and S61-113475 A disclose a solid golfball having an inner core where zinc acrylate as a co-crosslinkingagent, palmitic acid, stearic acid, or myristic acid as aco-crosslinking activator, zinc oxide as another co-crosslinkingactivator, and a reaction rate retarder are blended, with respect to 100parts by weight of a rubber.

Japanese Patent Publication No. S61-253079 A discloses a solid golf ballformed from a rubber composition containing an α,β-unsaturatedcarboxylic acid in an amount of 15 parts to 35 parts by weight, a metalcompound to react with the α,β-unsaturated carboxylic acid and form asalt thereof in an amount of 7 parts to 60 parts by weight, and a highfatty acid metal salt in an amount of 1 part to 10 parts by weight withrespect to 100 parts by weight of a base rubber.

Japanese Patent Publication No. 2008-212681 A discloses a golf ballcomprising, as a component, a molded and crosslinked product obtainedfrom a rubber composition essentially comprising a base rubber, afiller, an organic peroxide, an α,β-unsaturated carboxylic acid and/or ametal salt thereof, a copper salt of a saturated or unsaturated fattyacid.

Japanese Patent Publication No. 2008-523952 T discloses a golf ball, ora component thereof, molded from a composition comprising a baseelastomer selected from the group consisting of polybutadiene andmixtures of polybutadiene with other elastomers, at least one metallicsalt of an unsaturated monocarboxylic acid, a free radical initiator,and a non-conjugated diene monomer.

Japanese Patent Publication No. 2009-119256 A discloses a method ofmanufacturing a golf ball, comprising preparing a masterbatch of anunsaturated carboxylic acid and/or a metal salt thereof by mixing theunsaturated carboxylic acid and/or the metal salt thereof with a rubbermaterial ahead, using the masterbatch to prepare a rubber compositioncontaining the rubber material, and employing a heated and moldedproduct of the rubber composition as a golf ball component, wherein themasterbatch of the unsaturated carboxylic acid and/or the metal saltthereof comprises; (A) from 20 wt % to 100 wt % of a modifiedpolybutadiene obtained by modifying a polybutadiene having a vinylcontent of from 0 to 2%, a cis-1,4 bond content of at least 80% andactive terminals, the active terminal being modified with at least onetype of alkoxysilane compound, and (B) from 80 wt % to 0 wt % of a dienerubber other than (A) the above rubber component [the figures arerepresented by wt % in the case that a total amount of (A) and (B) equalto 100 wt %] and (C) an unsaturated carboxylic acid and/or a metal saltthereof.

For example, Japanese Patent Publications Nos. H6-154357 A, 2008-194471A, 2008-194473 A and 2010-253268 A disclose a core having a hardnessdistribution. Japanese Patent Publication No. H6-154357 A discloses atwo-piece golf ball comprising a core formed of a rubber compositioncontaining a base rubber, a co-crosslinking agent, and an organicperoxide, and a cover covering said core, wherein the core has thefollowing hardness distribution according to JIS-C type hardness meterreadings: (1) hardness at center: 58-73, (2) hardness at 5 to 10 mm fromcenter: 65-75, (3) hardness at 15 mm from center: 74-82, (4) surfacehardness: 76-84, wherein hardness (2) is almost constant within theabove range, and the relation (1)<(2)<(3)≤(4) is satisfied.

Japanese Patent Publication No. 2008-194471 A discloses a solid golfball comp sing a solid core and a cover layer that encases the core,wherein the solid core is formed of a rubber composition composed of 100parts by weight of a base rubber that includes from 60 to 100 parts byweight of a polybutadiene rubber having a cis-1,4 bond content of atleast 60% and synthesized using a rare-earth catalyst, from 0.1 to 5parts by weight of an organic sulfur compound, an unsaturated carboxylicacid or a metal salt thereof, an inorganic filler, and an antioxidant;the solid core has a deformation from 2.0 mm to 4.0 mm, when applying aload from an initial load of 10 kgf to a final load of 130 kgf and hasthe hardness distribution shown in the following table.

TABLE 1 Hardness distribution in solid core Shore D harness Center 30 to48 Region located 4 mm from center 34 to 52 Region located 8 mm fromcenter 40 to 58 Region located 12 mm from center (Q) 43 to 61 Regionlocated 2 to 3 mm inside of surface (R) 36 to 54 Surface (S) 41 to 59Hardness difference [(Q)-(S)]  1 to 10 Hardness difference [(S)-(R)]  3to 10

Japanese Patent Publication No. 2008-194473 A discloses a solid golfball comprising a solid core and a cover layer that encases the core,wherein the solid core is formed of a rubber composition composed of 100parts by weight of a base rubber that includes from 60 to 100 parts byweight of a polybutadiene rubber having a cis-1,4 bond content of atleast 60% and synthesized using a rare-earth catalyst, from 0.1 to 5parts by weight of an organic sulfur compound, an unsaturated carboxylicacid or a metal salt thereof, and an inorganic filler; the solid corehas a deformation from 2.0 mm to 4.0 mm, when applying a load from aninitial load of 10 kgf to a final load of 130 kgf and has the hardnessdistribution shown in the following table.

TABLE 2 Hardness distribution in solid core Shore D harness Center 25 to45 Region located 5 to 10 mm from center 39 to 58 Region located 15 mmfrom center 36 to 55 Surface (S) 55 to 75 Hardness difference 20 to 50between center and surface

Japanese Patent Publication No. 2010-253268 A discloses a multi-piecesolid golf ball comprising a core, an envelope layer encasing the core,an intermediate layer encasing the envelope layer, and a cover whichencases the intermediate layer and has formed on a surface thereof aplurality of dimples, wherein the core is formed primarily of a rubbermaterial and has a hardness which gradually increases from a center to asurface thereof, the hardness difference in JIS-C hardness units betweenthe core center and the core surface being at least 15 and, letting (I)be the average value for cross-sectional hardness at a position about 15mm from the core center and at the core center and letting (II) be thecross-sectional hardness at a position about 7.5 mm from the corecenter, the hardness difference (I)−(II) in JIS-C units being within ±2;and the envelope layer, intermediate layer and cover have hardness whichsatisfy the condition: cover hardness>intermediate layerhardness>envelope layer hardness.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball traveling agreat flight distance on driver shots.

The present invention provides a golf ball having a spherical core andat least one cover layer covering the spherical core, wherein thespherical core has an absorbance difference between absorbance measuredat a surface thereof and absorbance measured at a center thereof of0.024 or more, with respect to an absorption peak appeared at 1560±30cm⁻¹ when measuring the spherical core with a Fourier transform infraredspectrophotometer (hereinafter, sometimes simply referred to as“FT-IR”). The inventors of the present invention have made the presentinvention based on the findings that when measuring the surface of thespherical core and the center thereof with a Fourier transform infraredspectrophotometer, the absorption peak at 1560±30 cm⁻¹ strongly appearsat a spherical core central part where the hardness is low and theabsorption peak at 1560±30 cm⁻¹ weakly appears at a spherical coresurface part where the hardness is high. That is, if the spherical corehas the absorbance difference of the absorption peak between the surfaceand center thereof of 0.024 or more, the spherical core has anouter-hard inner-soft structure. As a result, the spin rate on drivershots is decreased, and the golf ball traveling a great distance ondriver shots is obtained.

In a preferred embodiment of the present invention, the spherical coreis formed from a rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator and (d) a carboxylic acid and/or a salt thereof, provided thatthe rubber composition further contains (e) a metal compound in case ofcontaining only (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent.

It is conceivable that the absorption peak appeared at 1560±30 cm⁻¹ isthe absorption peak attributed to a carboxylate ion in the rubbercomposition, when measuring the spherical core with a Fourier transformsinfrared spectrophotometer. As described above, this absorption peakthat is conceivably attributed to the carboxylate ion appears stronglyat the spherical core central part where the hardness is low, andappears weakly at the spherical core surface part where the hardness ishigh. That is, it is conceivable that the spherical core has a highconcentration of the carboxylate ion at the central part thereof wherethe hardness is low, and has a low concentration of the carboxylate ionat the surface part thereof where the hardness is high.

The reason why the absorption peak that is conceivably attributed to thecarboxylate ion appears strongly at the central part of the sphericalcore where the hardness is low, and appears weakly at the surface partof the spherical core where the hardness is high is considered asfollows. It is considered that the metal salt of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms blended in the rubbercomposition forms an ion cluster in the core, resulting in a metalcrosslinking of a rubber molecular chain. By blending (d) the carboxylicacid and/or the salt thereof with the rubber composition, (d) thecarboxylic acid and/or the salt thereof exchanges a cation with the ioncluster formed by the metal salt of (b) the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms, thereby breaking the metal crosslinkingby the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to8 carbon atoms. The cation exchange reaction easily occurs at the corecentral part where the temperature is high, and less occurs toward thecore surface. When molding the core, the internal temperature of thecore is high at the core central part and decreases toward the coresurface, since reaction heat from a curing reaction of the rubbercomposition accumulates at the core central part. That is, the breakingof the metal crosslinking easily occurs at the core central part, butless occurs toward the surface. As a result, it is conceivable thatsince a crosslinking density in the core increases from the center ofthe core toward the surface thereof, the core hardness increases fromthe center of the core toward the surface thereof. As a result of thesecation exchange reactions, it is conceivable that the concentration ofthe carboxylate ion is high at the spherical core central part and islow at the spherical core surface part, thereby causing a difference inabsorbance between the surface and the center of the spherical core.

The present invention provides a golf ball traveling a great flightdistance on driver shots.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing the golf ballaccording to the preferable embodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the sphericalcore;

FIG. 3 is a graph showing the hardness distribution of the sphericalcore;

FIG. 4 is a graph showing the hardness distribution of the sphericalcore;

FIG. 5 is a graph showing the hardness distribution of the sphericalcore;

FIG. 6 is a graph showing the hardness distribution of the sphericalcore;

FIG. 7 is a graph showing the hardness distribution of the sphericalcore;

FIG. 8 is a graph showing the hardness distribution of the sphericalcore;

FIG. 9 is a graph showing the hardness distribution of the sphericalcore;

FIG. 10 is a graph showing the hardness distribution of the sphericalcore;

FIG. 11 is a graph showing the hardness distribution of the sphericalcore;

FIG. 12 is a graph showing the hardness distribution of the sphericalcore;

FIG. 13 is a graph showing the hardness distribution of the sphericalcore;

FIG. 14 is a graph showing the hardness distribution of the sphericalcore;

FIG. 15 is a graph showing the hardness distribution of the sphericalcore;

FIG. 16 is a graph showing the hardness distribution of the sphericalcore;

FIG. 17 is a graph showing the hardness distribution of the sphericalcore;

FIG. 18 is a graph showing the hardness distribution of the sphericalcore;

FIG. 19 is a graph showing the hardness distribution of the sphericalcore;

FIG. 20 is a graph showing the hardness distribution of the sphericalcore;

FIG. 21 is a graph showing the hardness distribution of the sphericalcore;

FIG. 22 is a graph showing the hardness distribution of the sphericalcore;

FIG. 23 is a graph showing the hardness distribution of the sphericalcore; and

FIG. 24 is a FT-IR spectrum of the spherical core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball having a spherical core andat least one cover layer covering the spherical core, wherein thespherical core has an absorbance difference between absorbance measuredat a surface thereof and absorbance measured at a center thereof of0.024 or more, with respect to an absorption peak appeared at 1560±30cm⁻¹ when measuring the spherical core with a Fourier transform infraredspectrophotometer. The absorption peak at 1560±30 cm⁻¹ is the absorptionpeak of the carboxylate ion. The occurrence of the absorbance differencebetween the spherical core surface part and the spherical core centerpart means that the concentration of the carboxylate on is differentbetween the surface part of the spherical core and the central partthereof.

In the present invention, the absorbance difference between absorbancemeasured at the surface of the spherical core and absorbance measured atthe center thereof is preferably 0.025 or more, and more preferably0.026 or more. If the absorbance difference is large, the spherical corehas a higher degree of the outer-hard inner-soft structure. The golfball using the core with the higher degree of the outer-hard inner-softstructure provides the lower spin rate on driver shots and the greatflight distance. There is no limitation on the upper limit of theabsorbance difference between absorbance measured at the spherical coresurface and absorbance measured at the spherical core center, but theupper limit is preferably 0.600, more preferably 0.550, and even morepreferably 0.500. If the absorbance difference is too large, thedurability may deteriorate. The method of measuring the surface part andthe central part of the spherical core with a Fourier transform infraredspectrophotometer will be explained later.

In a preferred embodiment of the present invention, the spherical coreis formed from a rubber composition containing (a) a base rubber, (b) anunsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metalsalt thereof as a co-crosslinking agent, (c) a crosslinking initiatorand (d) a carboxylic acid and/or a salt thereof, provided that therubber composition further contains (e) a metal compound in case ofcontaining only (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent.

First, (a) the base rubber used in the present invention will beexplained. As (a) the base rubber used in the present invention, naturalrubber and/or synthetic rubber can be used. For example, polybutadienerubber, natural rubber, polyisoprene rubber, styrene polybutadienerubber, ethylene-propylene-diene rubber (EPDM), or the like can be used.These rubbers may be used solely or two or more of these rubbers may beused in combination. Among them, typically preferred is the highcis-polybutadiene having a cis-1,4 bond in a proportion of 40% or more,more preferably 80% or more, even more preferably 90% or more in view ofits superior resilience property.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in a contentof 2 mass % or less, more preferably 1.7 mass % or less, and even morepreferably mass % or less, if the content of 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cis polybutadiene preferably includes one synthesized using arare earth element catalyst. When a neodymium catalyst, which employs aneodymium compound of a lanthanum series rare earth element compound, isused, a polybutadiene rubber having a high content of a cis-1,4 bond anda low content of a 1,2-vinyl bond is obtained with excellentpolymerization activity. Such a polybutadiene rubber is particularlypreferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or more, more preferably 32 or more, even morepreferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140 or less, more preferably 120 or less, even morepreferably 100 or less, and most preferably 80 or less. It is noted thatthe Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is avalue measured according to JIS K6300 using an L rotor under theconditions of: a preheating time of 1 minute; a rotor revolution time of4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cis polybutadiene is excessively low, the processability maydeteriorate. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(manufactured by Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

Next, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof will be explained. (b) The α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofis blended as a co-crosslinking agent in the rubber composition and hasan action of crosslinking a rubber molecule by graft polymerization to abase rubber molecular chain. In the case that the rubber compositionused in the present invention contains only the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent,the rubber composition preferably further contains (e) a metal compoundas an essential component. Neutralizing the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms with the metal compound in the rubbercomposition provides substantially the same effect as using the metalsalt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.Further, in the case of using the metal salt of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms or using the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and the metal salt thereof incombination, (e) the metal compound may be used as an optionalcomponent.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes,for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid,crotonic acid, and the like.

Examples of the metals constituting the metal salts of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include:monovalent metal ions such as sodium, potassium, lithium or the like;divalent metal ions such as magnesium, calcium, zinc, barium, cadmium orthe like; trivalent metal ions such as aluminum ion or the like; andother metal ions such as zirconium or the like. The above metal ions canbe used solely or as a mixture of at least two of them. Among thesemetal ions, divalent metal ions such as magnesium, calcium, zinc,barium, cadmium or the like are preferable. Use of the divalent metalsalts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomseasily generates a metal crosslinking between the rubber molecules.Especially, as the divalent metal salt, zinc acrylate is preferable,because the zinc acrylate enhances the resilience of the resultant golfball. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or a metal salt thereof may be used solely or in combination atleast two of them.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and ispreferably 50 parts by mass or less, more preferably 45 parts by mass orless, even more preferably 35 parts by mass or less, with respect to 100parts by mass of (a) the base rubber, if the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof is less than 15 parts by mass, the content of (c) theco-crosslinking initiator which will be explained below must beincreased in order to obtain the appropriate hardness of theconstituting member formed from the rubber composition, which tends tocause the lower resilience. On the other hand, if the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof exceeds 50 parts by mass, the constituting memberformed from the rubber composition becomes excessively hard, which tendsto cause the lower shot feeling.

(c) The crosslinking initiator is blended in order to crosslink (a) thebase rubber component. As (c) the crosslinking initiator, an organicperoxide is preferred. Specific examples of the organic peroxide includeorganic peroxides such as dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Theseorganic peroxides may be used solely or two or more of these organicperoxides may be used in combination. Among them, dicumyl peroxide ispreferably used.

The content of (c) the crosslinking initiator is preferably 0.2 part bymass or more, and more preferably 0.5 part by mass or more, and ispreferably 5.0 parts by mass or less, and more preferably 2.5 parts bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the content of (c) the crosslinking initiator is less than 0.2 partby mass, the constituting member formed from the rubber compositionbecomes too soft, and thus the golf ball may have the lower resilience.If the content of (c) the crosslinking initiator exceeds 5.0 parts bymass, the amount of (b) the co-crosslinking agent must be decreased inorder to obtain the appropriate hardness of the constituting memberformed from the rubber composition, resulting in the insufficientresilience and lower durability of the golf ball.

Next, (d) the carboxylic acid and/or the salt thereof will be explained.It is conceivable that (d) the carboxylic acid and/or the salt thereofhas an action of breaking the metal crosslinking by the metal salt of(b) the α,β-unsaturated carboxylic acid at the center part of the core,when molding the core.

(d) The carboxylic acid and/or the salt thereof may include any one ofan aliphatic carboxylic acid (sometimes may be merely referred to as“fatty acid” in the present invention) and/or a salt thereof or anaromatic carboxylic acid and/or a salt thereof, but the aliphaticcarboxylic acid and/or the salt thereof is preferred. As the carboxylicacid and/or the salt thereof, a carboxylic acid having 4 to 30 carbonatoms and/or a salt thereof is preferred, and a carboxylic acid having 5to 25 carbon atoms and/or a salt thereof is more preferred. (d) Thecarboxylic acid and/or the salt thereof does not include (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof as the co-crosslinking agent.

The fatty acid may be either a saturated fatty acid or an unsaturatedfatty acid; however, a saturated fatty acid is preferable. Specificexamples of the saturated fatty acid (IUPAC name) are butanoic acid(C4), pentanoic acid (C5), hexanoic acid (C6), heptanoic acid (C7),octanoic acid (C8), nonanoic acid (C9), decanoic acid (C10), undecanoicacid (C11), dodecanoic acid (C12), tridecanoic acid (C13), tetradecanoicacid (C14), pentadecanoic acid (C15), hexadecanoic acid (C16),heptadecanoic acid (C17), octadecanoic acid (C18), nonadecanoic acid(C19), icosanoic acid (C20), henicosanoic acid (C21), docosanoic acid(C22), tricosanoic acid (C23), tetracosanoic acid (C24), pentacosanoicacid (C25), hexacosanoic acid (C26), heptacosanoic acid (C27),octacosanoic acid (C28), nonacosanoic acid (C29), and triacontanoic acid(C30).

Specific examples of the unsaturated fatty acid (IUPAC name) arebutenoic acid (C4), pentenoic acid (C5), hexenoic acid (C6), heptenoicacid (C7), octenoic acid (C8), nonenoic acid (C9), decenoic acid (C10),undecenoic acid (C11), dodecenoic acid (C12), tridecenoic acid (C13),tetradecenoic acid (C14), pentadecenoic acid (C15), hexadecenoic acid(C16), heptadecenoic acid (C17), octadecenoic acid (C18), nonadecenoicacid (C19), icosenoic acid (C20), henicosenoic acid (C21), docosenoicacid (C22), tricosenoic acid (C23), tetracosenoic acid (C24),penacosenoic acid (C25), hexacosenoic acid (C26), heptacosenoic acid(C27), octacosenoic acid (C28), nonacosenoic acid (C29), and,triacontenoic acid (C30).

Specific examples of the fatty acid (Common name) are, butyric acid(C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylicacid (C8), pelargonic acid (C9), capric acid (C10), lauric acid (C12),myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15),palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17),stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid(C18), linoleic acid (C18), linolenic acid (C18), 12-hydroxystearic acid(C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid(C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22),lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanicacid (C28), and melissic acid (C30). The fatty acid may be used alone oras a mixture of at least two of them. Among those described above capricacid, myristic acid, palmitic acid, stearic acid, behenic acid and oleicacid are preferable as the fatty acid.

There is no particular limitation on the aromatic carboxylic acid, aslong as it is a compound that has an aromatic ring and a carboxyl group.Specific examples of the aromatic carboxylic acids include, for example,benzoic acid (C7), phthalic acid (C8), isophthalic acid (C8),terephthalic acid (C8), hemimellitic acid (benzene-1,2,3-tricarboxylicacid) (C9), trimellitic acid (benzene-1,2,4-tricarboxylic acid) (C9),trimesic acid (benzene-1,3,5-tricarboxylic acid) (C9), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid) (C10), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid) (C10), pyromellitic acid(benzene-1,2,4,5-tetracarboxylic acid) (C10), mellitic acid (benzenehexacarboxylic acid) (C12), diphenic acid (biphenyl-2,2′-dicarboxylicacid) (C12), toluic acid (methylbenzoic acid) (C8), xylic acid (C9),prehnitylic acid (2,3,4-trimethylbenzoic acid) (C10), γ-isodurylic acid(2,3,5-trimethylbenzoic acid) (C10), durylic acid(2,4,5-trimethylbenzoic acid) (C10), β-isodurylic acid(2,4,6-trimethylbenzoic acid) (C10), α-isodurylic acid(3,4,5-trimethylbenzoic acid) (C10), cuminic acid (4-isopropylbenzoicacid) (C10), uvitic acid (5-methylisophthalic acid) (C9), α-toluic acid(phenylacetic acid) (C8), hydratropic acid (2-phenylpropanoic acid)(C9), and hydrocinnamic acid (3-phenylpropanoic acid) (C9).

Furthermore, examples of the aromatic carboxylic acids substituted witha hydroxyl group, an alkoxy group, or an oxo group include, for example,salicylic acid (2-hydroxybenzoic acid) (C7), anisic acid (methoxybenzoicacid) (C8), cresotinic acid (hydroxy(methyl)benzoic acid) (C8),o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid) (C8),m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid) (C8),p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid) (C8),o-pyrocatechuic acid (2,3-dihydroxybenzoic acid) (C7), β-resorcylic acid(2,4-dihydroxybenzoic acid) (C7), γ-resorcylic acid(2,6-dihydroxybenzoic acid) (C7), protocatechuic acid(3,4-dihydroxybenzoic acid) (C7), α-resorcylic acid(3,5-dihydroxybenzoic acid) (C7), vanillic acid(4-hydroxy-3-methoxybenzoic acid) (C8), isovanillic acid(3-hydroxy-4-methoxybenzoic acid) (C8), veratric acid(3,4-dimethoxybenzoic acid) (C9), o-veratric acid 2,3-dimethoxybenzoicacid) (C9), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid) (C8),m-hemipinic acid (4,5-dimethoxyphthalic acid) (C10), gallic acid(3,4,5-trihydroxybenzoic acid) (C7), syringic acid(4-hydroxy-3,5-dimethoxybenzoic acid) (C9), asaronic acid(2,4,5-trimethoxybenzoic acid) (C10), mandelic acid (hydroxy (phenyl)acetic acid) (C8), vanilmandelic acid (hydroxy (4-hydroxy-3-methoxyphenyl) acetic acid) (C9), homoanisic acid ((4-methoxy phenyl) aceticacid) (C9), homogentisic acid ((2,5-dihydroxyphenyl) acetic acid) (C8),homoprotocatechuic acid ((3,4-dihydroxyphenyl) acetic acid) (C8),homovanillic acid ((4-hydroxy-3-methoxy phenyl) acetic acid) (C9),homoisovanillic acid ((3-hydroxy-4-methoxy phenyl) acetic acid) (C9),homoveratric acid ((3,4-dimethoxy phenyl)acetic acid) (C10),o-homoveratric acid ((2,3-dimethoxy phenyl) acetic acid) (C10),homophthalic acid (2-(carboxymethyl) benzoic acid) (C9), homoisophthalicacid (3-(carboxymethyl) benzoic acid) (C9), homoterephthalic acid(4-(carboxymethyl) benzoic acid) (C9), phthalonic acid(2-(carboxycarbonyl) benzoic acid) (C9), isophthalonic acid(3-(carboxycarbonyl) benzoic acid) (C9), terephthaionic acid(4-(carboxycarbonyl) benzoic acid) (C9), benzilic acid(hydroxydiphenylacetic acid) (C14), atrolactic acid(2-hydroxy-2-phenylpropanoic acid) (C9), tropic acid(3-hydroxy-2-phenylpropanoic acid) (C9), melilotic acid(3-(2-hydroxyphenyl) propanoic acid) (C9), phloretic acid (3-(4-hydroxyphenyl) propanoic acid) (C9), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid) (C9), hydroferulic acid (3-(4-hydroxy-3-methoxy phenyl)propanoic acid) (C10), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl) propanoic acid) (C10), p-coumaric acid (3-(4-hydroxy phenyl)acrylic acid) (C9), umbellic acid (3-(2,4-dihydroxyphenyl) acrylic acid)(C9), caffeic acid (3-(3,4-dihydroxyphenyl) acrylic acid) (C9), ferulicacid (3-(4-hydroxy-3-methoxy phenyl) acrylic acid) (C10), isoferulicacid (3-(3-hydroxy-4-methoxy phenyl) acrylic acid) (C10), and sinapicacid (3-(4-hydroxy-3,5-dimethoxy phenyl) acrylic acid) (C11).

(d) The salt of the carboxylic acid includes the salt of the carboxylicacid mentioned above. The cation component of the salt, of thecarboxylic acid may be any one of a metal ion, an ammonium ion and anorganic cation. The metal ion includes monovalent metal ions such assodium, potassium, lithium, silver and the like; bivalent metal ionssuch magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel,manganese and the like; trivalent metal ions such as aluminum, iron andthe like; and other ions such as tin, zirconium, titanium and the like.These cation components may be used alone or as a mixture of at leasttwo of them.

The organic cation includes a cation having a carbon chain. The organiccation includes, for example, without limitation, an organic ammoniumion. Examples of the organic ammonium ion are: primary ammonium ionssuch as stearyl ammonium ion, hexyl ammonium ion, octyl ammonium ion,2-ethyl hexyl ammonium ion or the like; secondary ammonium ions such asdodecyl (lauryl) ammonium ion, octadecyl (stearyl) ammonium ion or thelike tertiary ammonium ions such as trioctyl ammonium ion or the like;and quaternary ammonium ion such as dioctyldimethyl ammonium ion,distearyldimethyl ammonium ion or the like. These organic cation may beused alone or as a mixture of at least two of them.

The content of (d) the carboxylic acid and/or the salt thereof may beappropriately determined to make an absorbance difference betweenabsorbance measured at a surface of the spherical core and absorbancemeasured at a center of the spherical core of 0.024 or more, withrespect to an absorption peak appeared at 1560±30 cm⁻¹. For example, thecontent of (d) the carboxylic acid and/or the salt thereof is preferably0.1 part by mass or more, more preferably 0.5 part by mass or more, evenmore preferably 1.0 part by mass or more, and is preferably 40.0 partsby mass or less, more preferably 30.0 parts by mass or less, even morepreferably 20.0 parts by mass or less with respect to 100 parts by massof (a) the base rubber. As described below, if a carboxylic acid havinga low carbon number and/or a salt thereof and a carboxylic acid having ahigh carbon number and/or a salt thereof are used in combination, it ispreferred that the total content of the carboxylic acid and/or the saltthereof falls within the above range.

If the content of (d) the carboxylic acid and/or the salt thereof is toolittle, the effect of adding (d) the carboxylic acid and/or the saltthereof is not sufficient, and thus the degree of the outer-hardinner-soft structure of the spherical core may be lowered. If thecontent is too much, the resilience of the core may be lowered, sincethe hardness of the resultant core may be lowered as a whole. There arecases where the surface of the zinc acrylate used as the co-crosslinkingagent is treated with the carboxylic acid and/or the salt thereof toimprove the dispersibility to the rubber. In the case of using zincacrylate whose surface is treated with the carboxylic acid and/or thesalt thereof, in the present invention, the amount of the carboxylicacid and/or the salt thereof used as a surface treating agent isincluded in the content of (d) the carboxylic acid and/or the saltthereof. For example, if 25 parts by mass of zinc acrylate whose surfacetreatment amount with the carboxylic acid and/or the salt thereof is 10mass % is used, the amount of the carboxylic acid and/or the saltthereof is 2.5 parts by mass and the amount of zinc acrylate is 22.5parts by mass. Thus, 2.5 parts by mass is counted as the content of (d)the carboxylic acid and/or the salt thereof.

It is preferable that the content of (d) the carboxylic acid and/or thesalt thereof is set appropriately by the kinds and combinations of thecarboxylic acid and/or the salt thereof used. In particular, it ispreferable that the content of (d) the carboxylic acid and/or the saltthereof is set appropriately in accordance with the number of carbonatoms and combinations of the carboxylic acid and/or the salt thereofused. It is conceivable that the action of breaking the metalcrosslinking by (d) the carboxylic acid and/or the salt thereof isaffected by the number of moles of the carboxylic acid and/or the saltthereof. Concurrently, the carboxylic acid and/or the salt thereof actsas a plasticizer for the spherical core. If the blending amount (mass)of (d) the carboxylic acid and/or the salt thereof to be addedincreases, the entire core is softened. This plasticizing effect isaffected by the blending amount (mass) of the carboxylic acid and/or thesalt thereof to be added. In view of those actions, on the same blendingamount (mass), the number of moles of the carboxylic acid and/or thesalt thereof to be added is made larger by using the carboxylic acidhaving less carbon atoms (small molecular weight) compared to using thecarboxylic acid having larger carbon atoms (large molecular weight).That is, the carboxylic acid having less carbon atoms and/or the saltthereof can enhance the effect of breaking the metal crosslinking, whilesuppressing softening the entire spherical core by the plasticizingeffect.

For example, when using a carboxylic acid having 1 to 14 carbon atoms,the content is preferably 0.1 part by mass or more, more preferably 0.5part by mass or more, even more preferably 1.0 part by mass or more, andis preferably 13.0 parts by mass or less, more preferably 12.0 parts bymass or less, even more preferably 11.0 parts by mass or less withrespect to 100 parts by mass of (a) the base rubber. When using a saltof a carboxylic acid having 1 to 14 carbon atoms, the content ispreferably 0.1 part by mass or more, more preferably 0.5 part by mass ormore, even more preferably 1.0 part by mass or more, and is preferably30.0 parts by mass or less, more preferably 25.0 parts by mass or lesseven more preferably 20.0 parts by mass or less with respect to 100parts by mass of (a) the base rubber.

In the case that the rubber composition used in the present inventioncontains only the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms as the co-crosslinking agent, the rubber composition furthercontains (e) a metal compound as an essential component. (e) The metalcompound is not limited as long as it can neutralize (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubbercomposition. (e) The metal compound includes, for example, metalhydroxides such as magnesium hydroxide, zinc hydroxide, calciumhydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide,copper hydroxide, and the like; metal oxides such as magnesium oxide,calcium oxide, zinc oxide, copper oxide, and the like; metal carbonatessuch as magnesium carbonate, zinc carbonate, calcium carbonate, sodiumcarbonate, lithium carbonate, potassium carbonate, and the like. Amongthese, (e) the metal compound preferably includes a divalent metalcompound, more preferably includes a zinc compound. The divalent metalcompound reacts with the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms, thereby forming a metal crosslinking. Use of the zinccompound provides a golf ball with excellent resilience. (e) These metalcompounds are used solely or as a mixture of at least two of them.

The rubber composition used in the present invention preferably furthercontains (f) an organic sulfur compound. By using (f) the organic sulfurcompound in addition to (d) the carboxylic acid and/or the salt thereoffor the rubber composition, the degree of the outer-hard and inner-softstructure of the core can be controlled. (f) The organic sulfur compoundis not particularly limited, as long as it is an organic compound havinga sulfur atom in the molecule thereof. Examples thereof include anorganic compound having a thiol group (—SH), a polysulfide bond having 2to 4 sulfur atoms (—S—S—, —S—S—S—, or —S—S—S—S—), or a metal saltthereof (—SM, —S-M-S—, —S-M-S—S—, —S—S-M-S—S—, S-M-S—S—S—, or the like;M is a metal atom). Furthermore, (f) the organic sulfur compound may beany one of aliphatic compounds (aliphatic thiol, aliphaticthiocarboxylic acid, aliphatic dithiocarboxylic acid, aliphaticpolysulfides, or the like), heterocyclic compounds, alicyclic compounds(alicyclic thiol, alicyclic thiocarboxylic acid, alicyclicdithiocarboxylic acid, alicyclic polysulfides, or the like), andaromatic compounds. (f) The organic sulfur compound includes, forexample, thiophenols, thionaphthols, polysulfides, thiocarboxylic acids,dithiocarboxylic acids, sulfenamides, thiurams, dithiocarbamates, andthiazoles. From the aspect of the larger hardness distribution of thecore, (f) the organic sulfur compound preferably includes, organiccompounds having a thiol group (—SH) or a metal salt thereof, morepreferably thiophenols, thionaphthols, or a metal salt thereof. Examplesof the metal salts are salts of monovalent metals such as sodium,lithium, potassium, copper (I), and silver (I), and salts of divalentmetals such as zinc, magnesium, calcium, strontium, barium, titanium(II), manganese (II), iron (II), cobalt (II), nickel(II), zirconium(II),and tin (II).

Examples of the thiophenols include, for example, thiophenol;thiophenols substituted with a fluoro group, such as 4-fluorothiophenol,2,5-difluorothiophenol, 2,4,5-trifluorothiophenol,2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol and the like;thiophenols substituted with a chloro group, such as 2-chlorothiophenol,4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol and the like;thiophenols substituted with a bromo group, such as 4-bromothiophenol,2,5-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like;thiophenols substituted with an iodo group, such as 4-iodothiophenol,2,5-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like; or ametal salt thereof. As the metal salt, zinc salt is preferred.

Examples of the naphthalenethiols (thionaphthols) are2-naphthalenethiol, 1-naphthalenethiol, 2-chloro-1-naphthalenethiol,2-bromo-1-naphthalenethiol, 2-fluoro-1-naphthalenethiol,2-cyano-1-naphthalenethiol, 2-acetyl-1-naphthalenethiol,1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol,1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and1-acetyl-2-naphthalenethiol and metal salts thereof. Preferable examplesinclude 1-naphthalenethiol, 2-naphthalenethiol and zinc salt thereof.

The sulfenamide based organic sulfur compound includes, for example,N-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. The thiuram based organic sulfurcompound includes, for example, tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.The dithiocarbamates include, for example, zinc dimethyldithiocarbamate,zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethylphenyl dithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. The thiazole based organic sulfur compoundincludes, for example, 2-mercaptobenzothiazole (MBT), dibenzothiazyldisulfide (MBTS), sodium salt, zinc salt, copper salt, orcyclohexylamine salt of 2-mercaptobenzothiazole, 2-(2,4-dinitrophenyl)mercaptobenzothiazole, and 2-(2,6-diethyl-4-morpholinothio)benzothiazole.

(f) The organic sulfur compound can be used solely or as a mixture of atleast two of them.

The content of (f) the organic sulfur compound is preferably 0.05 partby mass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe content of (f) the organic sulfur compound is less than 0.05 part bymass, the effect of adding (f) the organic sulfur compound cannot beobtained and thus the resilience may not improve. If the content of (f)the organic sulfur compound exceeds 5.0 parts by mass, the compressiondeformation amount of the obtained golf ball becomes large and thus theresidence may be lowered.

The rubber composition used in the present invention may includeadditives such as a pigment, a filler for adjusting weight or the like,an antioxidant, a peptizing agent, and a softener where necessary.Further, as described above, if the rubber composition used in thepresent invention contains only the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms as a co-crosslinking agent, the rubbercomposition preferably contains (e) the metal compound.

Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment. As the whitepigment, titanium oxide is preferably used. The type of titanium oxideis not particularly limited, but rutile type is preferably used becauseof the high opacity. The blending amount of titanium oxide is preferably0.5 part by mass or more, and more preferably 2 parts by mass or more,and is preferably 8 parts by mass or less, and more preferably 5 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber.

It is also preferred that the rubber composition contains both a whitepigment and a blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. Examples of the purplepigment include anthraquinone violet, dioxazine violet, and methylviolet.

The blending amount of the blue pigment is preferably 0.001 part by massor more, and more preferably 0.05 part by mass or more, and ispreferably 0.2 part by mass or less, and more preferably 0.1 part bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the blending amount of the blue pigment is less than 0.001 part bymass, blueness is insufficient, and the color looks yellowish. If theblending amount of the blue pigment exceeds 0.2 part by mass, bluenessis excessively strong, and a vivid white appearance is not provided.

The filler blended in the rubber composition is used as a weightadjusting agent for mainly adjusting the weight of the golf ballobtained as a final product. The filler may be blended where necessary.The filler includes, for example, inorganic fillers such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide, tungsten powder,molybdenum powder, or the like. The content of the filler is preferably0.5 part by mass or more, more preferably 1 part by mass or more, and ispreferably 30 parts by mass or less, more preferably 25 parts by mass orless, even more preferably 20 parts by mass or less. If the content ofthe filler is less than 0.5 part by mass, it is difficult to adjust theweight, while if the content of the filler exceeds 30 parts by mass, theweight ratio of the rubber component is reduced and thus the resiliencetends to be lowered.

The blending amount of the antioxidant is preferably 0.1 part by mass ormore and 1 part by mass or less, with respect to 100 parts by mass of(a) the base rubber. In addition, the blending amount of the peptizingagent is preferably 0.1 part by mass or more and 5 parts by mass orless, with respect to 100 parts by mass of (a) the base rubber.

The rubber composition used in the present invention is obtained bymixing and kneading (a) the base rubber, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal saltthereof, (c) the crosslinking initiator, and (d) the carboxylic acidand/or the salt thereof, and other additives where necessary. Thekneading can be conducted, without any limitation, with a well-knownkneading machine such as a kneading roll, a Banbury mixer, a kneader, orthe like.

The spherical core of the golf ball of the present invention can beobtained by molding the rubber composition after kneaded. Thetemperature for molding the spherical core is preferably 120° C. ormore, more preferably 150° C. or more, even more preferably 160° C. ormore, and is preferably 170° C. or less. If the molding temperatureexceeds 170° C., the surface hardness of the core tends to decrease. Themolding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The moldingtime preferably ranges from 10 minutes to 60 minutes.

In a preferable embodiment, when the hardness is measured at nine pointsobtained by dividing a radius of the spherical core into equal partshaving 12.5% interval and the hardness is plotted against distance (%)from the center of the spherical core, the spherical core is such thatR² of a linear approximation curve obtained by the least square methodis 0.95 or higher. If R² is 0.95 or more, the linearity of the corehardness distribution is enhanced, thus the spin rate on driver shotsdecreases, resulting in the greater flight distance.

The hardness of the spherical core is JIS-C hardness measured at ninepoints obtained by dividing a radius of the spherical core into equalparts having 12.5% interval. That is, JIS-C hardness is measured at ninepoints, namely at distances of 0% (core center), 12.5%, 25% 37.5% 50%,62.5%, 75%, 87.5%, 100% (core surface) from the core center. Next, themeasurement results are plotted to make a graph having JIS-C hardness asa vertical axis and distances (%) from the core center as a horizontalaxis. In the present invention, R² of a linear approximation curveobtained from this graph by the least square method is preferably 0.95or higher. R² of the linear approximation curve obtained by the leastsquare method indicates the linearity of the obtained plot. In thepresent invention, R² of 0.95 or more means that the core has thehardness distribution where the hardness increases linearly or almostlinearly. If the core having the hardness distribution where thehardness increases linearly or almost linearly is used for the golfball, the spin rate on driver shots decrease. As a result, the flightdistance on driver shots increases. R² of the linear approximation curveis preferably 0.96 or more. The higher linearity provides a greaterflight distance on driver shots.

The spherical core preferably has a hardness difference (Hs−Ho) betweena surface hardness Hs and a center hardness Ho of 18 or more, morepreferably 20 or more, even more preferably 22 or more, and preferablyhas a hardness difference of 80 or less, more preferably 70 or less evenmore preferably 60 or less in JIS-C hardness. If the hardness differencebetween the center hardness and the surface hardness is large, the golfball having a great flight distance due to the high launch angle and lowspin rate is obtained. On the other hand, if the hardness difference istoo large, the durability of the obtained golf ball may be lowered.

The spherical core preferably has the center hardness Ho of 30 or more,more preferably 40 or more, even more preferably 45 or more in JIS-Chardness. If the center hardness Ho is less than 30 in JIS-C hardness,the core becomes too soft and thus the resilience may be lowered.Further, the spherical core preferably has the center hardness Ho of 70or less, more preferably 65 or less, even more preferably 60 or less inJIS-C hardness. If the center hardness Ho exceeds 70 in JIS-C hardness,the core becomes too hard and thus the shot feeling tends to be lowered.

The spherical core preferably has the surface hardness Hs of 76 or more,more preferably 78 or more, even more preferably 80 or more, andpreferably has the surface hardness Hs of 100 or less, more preferably95 or less in JIS-C hardness. If the surface hardness is 76 or more inJIS-C hardness, the spherical core does not become excessively soft, andthus the better resilience is obtained. Further, if the surface hardnessof the spherical core is 100 or less in JIS-C hardness, the sphericalcore does not become excessively hard, and thus the better shot feelingis obtained.

The spherical core preferably has the diameter of 34.8 mm or more, morepreferably 36.8 mm or more, and even more preferably 38.8 mm or more,and, preferably has the diameter of 42.2 mm or less, more preferably41.8 mm or less, and even more preferably 41.2 mm or less and mostpreferably 40.8 mm or less. If the spherical core has the diameter of34.8 mm or more, the thickness of the cover does not become too thickand thus the resilience becomes better. On the other hand if thespherical core has the diameter of 42.2 mm or less, the thickness of thecover does not become too thin, and thus the cover functions better.

When the spherical core has a diameter from 34.8 mm to 42.2 mm, acompression deformation amount (shrinking deformation amount of thespherical core along the compression direction) of the spherical corewhen applying a load from 98 N as an initial load to 1275 N as a finalload is preferably 2.0 mm or more, more preferably 2.8 mm or more, andis preferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling ofthe golf ball becomes better. If the compression deformation amount is6.0 mm or less, the resilience of the golf ball becomes better.

The golf ball cover of the present invention is formed from a covercomposition containing a resin component. Examples of the resincomponent include, for example, an ionomer rein: a thermoplasticpolyurethane elastomer having a commercial name of “Elastollan”commercially available from BASF Japan Ltd; a thermoplastic polyamideelastomer having a commercial name of “Pebax” commercially availablefrom Arkema K. K.; a thermoplastic polyester elastomer having acommercial name of “Hytrel” commercially available from Du Pont-TorayCo., Ltd.; and a thermoplastic styrene elastomer having a commercialname of “Rabalon” commercially available from Mitsubishi ChemicalCorporation; and the like.

The ionomer resin includes a product prepared by neutralizing at least apart of carboxyl groups in the binary copolymer composed of an olefinand an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with ametal ion, a product prepared by neutralizing at least a part ofcarboxyl groups in the ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and anα,β-unsaturated carboxylic acid ester with a metal ion, or a mixture ofthose. The olefin preferably includes an olefin having 2 to 8 carbonatoms. Examples of the olefin are ethylene, propylene, butene, pentene,hexene, heptene, and octene. The olefin more preferably includesethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleicacid and crotonic acid. Among these, acrylic acid and methacrylic acidare particularly preferred. Examples of the α,β-unsaturated carboxylicacid ester include methyl ester, ethyl ester, propyl ester, n-butylester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid,maleic acid or the like. In particular, acrylic acid ester andmethacrylic acid ester are preferable. Among these, the ionomer resinpreferably includes the metal ion-neutralized product of the binarycopolymer composed of ethylene and (meth)acrylic acid and the metalion-neutralized product of the ternary copolymer composed of ethylene,(meth)acrylic acid, and (meth)acrylic acid ester.

Specific examples of the ionomer resins include trade name “Himilan(registered trademark) (e.g. the binary copolymerized ionomer such asHimilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706(Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternarycopolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))”commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples include “Surlyn (registered trademark) (e.g. thebinary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn),Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn),Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li),Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), HPF1000 (Mg), HPF 2000 (Mg))” commercially available from E.I. du Pont deNemours and Company.

Further, examples include “Iotek (registered trademark) (e.g. the binarycopolymerized ionomer such as Iotek 8000 (Na), Iotek 8030 (Na), Iotek7010 (Zn), Iotek 7030 (Zn); and the ternary copolymerized ionomer suchas Iotek 7510 (Zn), Iotek 7520 (Zn))” commercially available fromExxonMobil Chemical Corporation.

It is noted that Na, Zn, Li, and Mg described in the parentheses afterthe trade names indicate metal types of neutralizing metal ions for theionomer resins.

The cover composition constituting the cover of the golf ball of thepresent invention preferably includes, as a resin component, athermoplastic polyurethane elastomer or an ionomer rein. In case ofusing the ionomer rein, it is preferred to use a thermoplastic styreneelastomer together. The content of the polyurethane or ionomer resin inresin component of the cover composition is preferably 50 mass % ormore, more preferably 60 mass % or more, and even more preferably 70mass % or more.

In the present invention, the cover composition may further contain apigment component such as a white pigment (for example, titanium oxide),a blue pigment, and a red pigment; a weight adjusting agent such as zincoxide, calcium carbonate, and barium sulfate; a dispersant; anantioxidant; an ultraviolet absorber; a light stabilizer; a fluorescentmaterial or a fluorescent brightener and the like, as long as they donot impair the effect of the present invention.

The amount of the white pigment (for example, titanium oxide) ispreferably 0.5 part or more, more preferably 1 part or more, and thecontent of the white pigment is preferably 10 parts or less, morepreferably 8 parts or less, with respect to 100 parts of the resincomponent constituting the cover by mass. If the amount of the whitepigment is 0.5 part by mass or more, it is possible to impart theopacity to the resultant cover. Further, if the amount of the whitepigment is more than 10 parts by mass, the durability of the resultantcover may deteriorate.

The slab hardness of the cover composition is preferably set inaccordance with the desired performance of the golf balls. For example,in case of a so-called distance golf ball which focuses on a flightdistance, the cover composition preferably has a slab hardness of 50 ormore, more preferably 55 or more, and preferably has a slab hardness of80 or less, more preferably 70 or less in shore D hardness. If the covercomposition has a slab hardness of 50 or more, the obtained golf ballhas a high launch angle and low spin rate on driver shots and ironshots, and thus the flight distance becomes large. If the covercomposition has a slab hardness of 80 or less, the golf ball excellentin durability is obtained. Further, in case of a so-called spin golfball which focuses on controllability, the cover composition preferablyhas a slab hardness of less than 50, and preferably has a slab hardnessof 20 or more, more preferably 25 or more in shore D hardness. If thecover composition has a slab hardness of less than 50, the flightdistance on driver shots can be improved by the core of the presentinvention, as well as the obtained golf ball readily stops on the greendue to the high spin rate on approach shots. If the cover compositionhas a slab hardness of 20 or more, the abrasion resistance improves. Incase of a plurality of cover layers, the slab hardness of the covercomposition constituting each layer can be identical or different, aslong as the slab hardness of each layer is within the above range.

An embodiment for molding a cover is not particularly limited, andincludes an embodiment which comprises injection molding the covercomposition directly onto the core, or an embodiment which comprisesmolding the cover composition into a hollow-shell, covering the corewith a plurality of the hollow-shells and subjecting the core with aplurality of the hollow shells to the compression-molding (preferably anembodiment which comprises molding the cover composition into a halfhollow-shell, covering the core with the two half hollow-shells, andsubjecting the core with the two half hollow-shells to thecompression-molding).

When molding the cover in a compression molding method, molding of thehalf shell can be performed by either compression molding method orinjection molding method, and the compression molding method ispreferred. The compression-molding of the cover composition into halfshell can be carried out, for example, under a pressure of 1 MPa or moreand 20 MPa or less at a temperature of −20° C. or more and 70° C. orless relative to the flow beginning temperature of the covercomposition. By performing the molding under the above conditions, ahalf shell having a uniform thickness can be formed. Examples of amethod for molding the cover using half shells include compressionmolding by covering the core with two half shells. The compressionmolding of half shells into the cover can be carried out, for example,under a pressure of 0.5 MPa or more and 25 MPa or less at a temperatureof −20° C. or more and 70° C. or less relative to the flow beginningtemperature of the cover composition. By performing the molding underthe above conditions, a golf ball cover having a uniform thickness canbe formed.

In the case of directly injection molding the cover composition, thecover composition extruded in the pellet form beforehand may be used forinjection molding or the materials such as the base resin components andthe pigment may be dry blended, followed by directly injection moldingthe blended material. It is preferred to use upper and lower moldshaving a spherical cavity and pimples for forming, a cover, wherein apart of the pimples also serves as a retractable hold pin. When moldingthe cover by injection molding, the hold pin is protruded to hold thecore, and the cover composition which has been heated and melted ischarged and then cooled to obtain a cover. For example, it is preferredthat the cover composition heated and melted at the temperature rangingfrom 200° C. to 250° C. is charged into a mold held under the pressureof 9 MPa to 15 MPa for 0.5 to 5 seconds, and after cooling for 10 to 60seconds, the mold is opened and the golf ball with the cover molded istaken out from the mold.

The concave portions called “dimple” are usually formed on the surfaceof the cover. The total number of the dimples is preferably 200 or moreand 500 or less. If the total number is less than 200, the dimple effectis hardly obtained. On the other hand if the total number exceeds 500,the dimple effect is hardly obtained because the size of the respectivedimples is small. The shape (shape in a plan view) of dimples includes,for example, without limitation, a circle, polygonal shapes such asroughly triangular shape, roughly quadrangular shape, roughly pentagonalshape, roughly hexagonal shape, and another irregular shape. The shapeof the dimples is employed solely or at least two of them may be used incombination.

In the present invention, the thickness of the cover of the golf ball ispreferably 4.0 mm or less, more preferably 3.0 mm or less, even morepreferably 2.0 mm or less. If the thickness of the cover is 4.0 mm orless, the resilience and shot feeling of the obtained golf ball becomebetter. The thickness of the cover is preferably 0.3 mm or more, morepreferably 0.5 mm or more, and even more preferably 0.8 mm or more, andmost preferably 1.0 mm or more. If the thickness of the cover is lessthan 0.3 mm, the durability and the wear resistance of the cover maydeteriorate. If the cover has a plurality of layers, it is preferredthat the total thickness of the cover layers falls within the aboverange.

After the cover is molded, the mold is opened and the golf ball body istaken out from the mold, and as necessary, the golf ball body ispreferably subjected to surface treatments such as deburring, cleaning,and sandblast. If desired, a paint film or a mark may be formed. Thepaint film preferably has a thickness of, but not limited to, 5 μm orlarger, and more preferably 7 μm or larger, and preferably has athickness of 50 μm or smaller, and more preferably 40 μm or smaller,even more preferably 30 μm or smaller. If the thickness is smaller than5 μm, the paint film is easy to wear off due to continued use of thegolf ball, and if the thickness is larger than 50 μm, the effect of thedimples is reduced, resulting in lowering flying performance of the golfball.

When the golf ball of the present invention has a diameter in a rangefrom 40 mm to 45 mm, a compression deformation amount of the golf ball(shrinking amount of the golf ball in the compression direction thereof)when applying a load from an initial load of 98 N to a final load of1275 N to the golf ball is preferably 2.0 mm or more, more preferably2.4 mm or more, even more preferably 2.5 mm or more, most preferably 2.8mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm orless. If the compression deformation amount is 2.0 mm or more, the golfball does not become excessively hard, and thus exhibits the good shotfeeling. On the other hand, if the compression deformation amount is 5.0mm or less, the resilience is enhanced.

The golf ball construction is not limited as long as the golf ball ofthe present invention comprises a spherical core and at least one coverlayer covering the spherical core. FIG. 1 is a partially cutawaysectional view showing the golf ball 2 according to the preferableembodiment of the present invention. The golf ball 2 comprises aspherical core 4, and a cover 12 covering the spherical core 4.Plurality of dimples 14 are formed on a surface of the cover. Otherportions than dimples 14 on the surface of the golf ball 2 are land 16.The golf ball 2 is provided with a paint layer and a mark layer outsidethe cover 12, but these layers are not depicted.

The spherical core preferably has a single layered structure. Unlike themulti-layered structure, the spherical core of the single layeredstructure does not have an energy loss at the interface of themulti-layered structure when hitting, and thus has an improvedresilience. The cover has a structure of at least one layer, for examplea single layered structure, or a multi-layered structure of at least twolayers. The golf ball of the present invention includes, for example, atwo-piece golf ball comprising a spherical core and a single layeredcover disposed around the spherical core, a multi-piece golf ballcomprising a spherical core, and at least two cover layers disposedaround the spherical core (including the three-piece golf ball), and awound golf ball comprising a spherical core, a rubber thread layer whichis formed around the spherical core, and a cover disposed over therubber thread layer. The present invention can be suitably applied toany one of the above golf balls.

EXAMPLES

Hereinafter, the present invention will be described in detail by way ofexample. The present invention is not limited to examples describedbelow. Various changes and modifications can be made without departingfrom the spirit and scope of the present invention.

[Evaluation Methods]

(1) Compression Deformation Amount (mm)

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball in the compression direction thereof),when applying a load from 98 N as an initial load to 1275 N as a finalload to the core or golf ball, was measured.

(2) Coefficient of Restitution

A 198.4 g of metal cylindrical object was allowed to collide with eachcore or golf ball at a speed of 40 m/sec, and the speeds of thecylindrical object and the core or golf ball before and after thecollision were measured. Based on these speeds and the mass of eachobject, coefficient of restitution for each core or golf ball wascalculated. The measurement was conducted by using twelve samples foreach core or golf ball, and the average value was regarded as thecoefficient of restitution for the core or golf ball. In tables 3, 5 to7, the coefficient of restitution of golf balls are shown as thedifference from that of the golf ball (core) No. 6. In tables 4, thecoefficient of restitution of golf balls are shown as the differencefrom that of the golf ball (core) No. 13.

(3) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition, and stored at 23 for two weeks. Three or more ofthese sheets were stacked on one another so as not to be affected by themeasuring substrate on which the sheets were placed, and the hardness ofthe stack was measured with a type P1 auto loading durometermanufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D typespring hardness tester prescribed in ASTM-D2240.

(4) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co.,Ltd., provided with a JIS-C type spring hardness tester was used tomeasure the hardness of the spherical core. The hardness measured at thesurface of the spherical core was adopted as the surface hardness of thespherical core. The spherical core was cut into two hemispheres toobtain a cut plane, and the hardness were measured at the central pointand at predetermined distances from the central point. The core hardnesswere measured at 4 points at predetermined distances from the centralpoint of the cut plane of the core. The core hardness was calculated byaveraging the hardness measured at 4 points.

(5) Initial Ball Speed (m/s), Flight Distance (m), and Spin Rate (rpm)

A metal-head W#1 driver (XXIO, shaft S. Loft angle: 11° available fromSRI Sports Limited) was installed on a swing robot available from GolfLaboratories, Inc. Golf balls were hit at a head speed of 40 m/sec., andthe spin rate of the golf balls right after hitting and the flightdistance (the distance from the launch point to the stop point) weremeasured. This measurement was conducted twelve times for each golfball, and the average values were adopted as the measurement value forthe golf ball. A sequence of photographs of the hit golf ball were takenfor measuring the initial speed, and the spin rate right after hittingthe golf ball. In tables 3, 5 to 7, the flight distance and the spinrate on the driver shots of golf balls are shown as the difference fromthose of the golf ball (core) No. 6. In table 4, the flight distance andthe spin rate on the driver shots of golf balls are shown as thedifference from those of golf ball (core) No. 13.

(6) FT-IR Measurement

A circle-shaped measuring sample having a thickness of 5 mm was cut outof the spherical core so that the plane including two poles of thespherical core (hereinafter, may be referred to as “pole plane”) becomescenter of the thickness with a bandsaw (ANDOSAW TA-300). The surface ofthe circle-shaped measuring sample was sandpapered with a sandpaper(around #240) and ground with a surface grinder (available from OkamotoMachine Tool Works, Ltd.) to have a thickness of 4 mm. The circle-shapedsample having a thickness of 4 mm was cut into the two parts (upper andlower parts) in a thickness direction to obtain the two circle-shapedmeasuring samples having the thickness of 2 mm. When cutting thespherical core, the spherical core was made wet with a mixing solutionof water and isopropyl alcohol (water:isopropyl alcohol=8:2) to improvethe cutting with the blade. The obtained circle-shaped measuring sampleshaving the thickness of 2 mm were analyzed with FT-IR at thecircumferential edge part (spherical core surface) and the central part(spherical core center) of the inside cross-sectional surface (poleplane side), respectively. The circumferential edge part is a part whichis an, edge part as close to the circumference as possible. Themeasurement, with FT-IR was conducted with an Infrared spectrophotometer(Auto IMAGE FT-IR) available from Perkin Elmer. Inc. by a macro ATRmethod (diamond prism, observation diameter: about 1 mm) at theconditions of resolution: 4 cm⁻¹ and cumulative scan number: 4 times.Four circle-shaped measuring samples prepared from two spherical coreswere used for FT-IR measurement and subjected to averaging procedure toobtain the respective spectra of the spherical core center and thespherical core surface. The difference spectrum was prepared from thespectrum of the spherical core center and the spectrum of the sphericalcore surface to read the height of the absorption peak at 1560±30 cm⁻¹.It is noted that the difference spectrum was prepared to make the peak(910 cm⁻¹) attributed to the vinyl bond of the polybutadiene flat.

[Production of Golf Balls]

(1) Production of Cores

The rubber compositions having formulations shown in Tables 3 to 7 werekneaded and heat-pressed in upper and lower molds, each having ahemispherical cavity, at 170 for 20 minutes to prepare spherical coreshaving a diameter of 39.8 mm.

TABLE 3 Golf ball No. 1 2 3 4 5 6 7 Rubber composition BR730 100 100 100100 100 100 100 (parts by mass) Sanceler SR 28 28 30 29 29 23 30 Zincoxide 5 5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) *1)2-Thionaphthol 0.32 0.32 0.32 0.32 0.32 — 0.32 Stearic acid 10 — — — — —— Zinc stearate — 10 — — — — — Myristic acid — — 10 — — — — Zincmyristate — — — 10 — — — Zinc octanoate — — — — 5 — — Dicumyl peroxide0.8 0.8 0.8 0.8 0.8 0.8 0.8 Total amount of carboxylic acid/salt 12.812.8 13.0 12.9 7.9 2.3 3.0 Core hardness distribution Center hardness54.2 54.4 49.6 50.6 47.6 57.7 56.5 (JIS-C) 12.5% point hardness 58.260.2 53.0 54.4 54.2 63.2 62.0 25% point hardness 62.5 64.4 56.5 58.657.9 66.5 65.9 37.5% point hardness 65.1 67.2 58.9 62.0 60.5 67.7 67.050% point hardness 65.9 68.3 60.0 63.4 62.5 67.7 66.8 62.5% pointhardness 70.0 70.5 66.1 67.8 68.5 68.2 66.5 75% point hardness 77.7 77.674.5 73.5 75.1 73.5 73.4 87.5% point hardness 80.7 80.6 77.1 78.7 78.576.1 79.0 Surface hardness 83.6 83.9 81.0 82.4 81.6 81.4 84.1 Surfacehardness − center hardness 29.4 29.5 31.4 31.8 34.0 23.7 27.6 R² ofapproximated curve 0.98 0.98 0.97 0.99 0.99 0.92 0.89 Slope ofapproximated curve 0.29 0.28 0.32 0.31 0.34 0.20 0.23 Core coefficientof restitution 0.016 0.007 0.012 0.001 −0.007 0.000 0.012 Corecompression deformation amount (mm) 3.94 3.83 4.12 4.37 4.58 4.29 4.06Peak intensity (1560 ± 30 cm⁻¹) of difference 0.037 0.029 0.065 0.1030.057 0.011 0.010 spectrum between the center and surface of the coreCover composition A A A A A A A Cover hardness (Shore D) 65 65 65 65 6565 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spinrate (rpm) −90 −80 −80 −80 −110 0 −10 Driver flying distance (m) 3.8 2.92.5 2.5 4 0 1.5 Coefficient of restitution 0.016 0.007 0.012 0.001−0.007 0 0.009 Compression deformation amount (nm) 3.24 3.13 3.42 3.673.88 3.59 3.36

TABLE 4 Golf ball No. 8 9 10 11 12 13 14 Rubber composition BR730 100100 100 100 100 100 100 (parts by mass) Sanceler SR 28 28 30 29 29 23 30Zinc oxide 5 5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) *1)2-Thionaphthol 0.32 0.32 0.32 0.32 0.32 — 0.32 Stearic acid 10 — — — — —— Zinc stearate — 10 — — — — — Myristic acid — — 10 — — — — Zincmyristate — — — 10 — — — Zinc octanoate — — — — 5 — — Dicumyl peroxide0.8 0.8 0.8 0.8 0.8 0.8 0.8 Total amount of carboxylic acid/salt 12.812.8 13.0 12.9 7.9 2.3 3.0 Core hardness distribution Center hardness54.2 54.4 49.6 50.6 47.6 57.7 56.5 (JIS-C) 12.5% point hardness 58.260.2 53.0 54.4 54.2 63.2 62.0 25% point hardness 62.5 64.4 56.5 58.657.9 66.5 65.9 37.5% point hardness 65.1 67.2 58.9 62.0 60.5 67.7 67.050% point hardness 65.9 68.3 60.0 63.4 62.5 67.7 66.8 62.5% pointhardness 70.0 70.5 66.1 67.8 68.5 68.2 66.5 75% point hardness 77.7 77.674.5 73.5 75.1 73.5 73.4 87.5% point hardness 80.7 80.6 77.1 78.7 78.576.1 79.0 Surface hardness 83.6 83.9 81.0 82.4 81.6 81.4 84.1 Surfacehardness − center hardness 29.4 29.5 31.4 31.8 34.0 23.7 27.6 R² ofapproximated curve 0.96 0.98 0.97 0.99 0.99 0.92 0.89 Slope ofapproximated curve 0.29 0.28 0.32 0.31 0.34 0.20 0.23 Core coefficientof restitution 0.016 0.007 0.012 0.001 −0.007 0.000 0.012 Corecompression deformation amount (mm) 3.94 3.83 4.12 4.37 4.58 4.29 4.06Peak intensity (1560 ± 30 cm⁻¹) of difference 0.037 0.029 0.065 0.1030.057 0.011 0.010 spectrum between the center and surface of the coreCover composition B B B B B B B Cover hardness (Shore D) 47 47 47 47 4747 47 Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spinrate (rpm) −90 −80 −70 −80 −110 0 −10 Driver flying distance (m) 3.8 2.92.7 2.5 4 0 1.5 Coefficient of restitution 0.016 0.007 0.011 0.001−0.007 0 0.009 Compression deformation amount (mm) 3.74 3.63 3.92 4.174.38 4.09 3.86

TABLE 5 Golf ball No. 15 16 17 18 19 20 Rubber composition BR730 100 100100 100 100 100 (parts by mass) Sanceler SR 29 23 29 29 29 29 Zinc oxide5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) 2-Thionaphthol — —0.1 — — 0.1 2,4,5-TCTP — — — 0.11 — — 2,6-DCTP 0.42 — — — 0.42 — Zincstearate 10 — — — — — zinc myristate — 10 10 10 10 — Zinc octanoate — —— — — 5 Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 Total amount ofcarboxylic acid/salt 12.9 12.3 12.9 12.9 12.9 7.9 Core hardnessdistribution Center hardness 49.9 46.6 51.0 51.6 48.4 46.2 (JIS-C) 12.5%point hardness 55.3 53.4 58.5 57.8 54.2 52.9 25% point hardness 62.159.6 63.9 63.9 60.8 60.0 37.5% point hardness 65.4 62.7 67.7 68.0 63.663.9 50% point Hardness 67.7 64.9 68.9 69.4 65.2 65.8 62.5% pointhardness 68.0 69.4 69.8 70.2 65.8 68.4 75% point hardness 73.9 72.8 76.576.6 73.8 75.9 87.5% point hardness 77.8 73.0 78.8 78.4 77.2 76.0Surface hardness 83.2 78.3 83.7 84.3 82.6 83.7 Surface hardness − centerhardness 33.3 31.7 32.7 32.7 34.2 37.5 R² of approximated curve 0.970.96 0.96 0.97 0.97 0.98 Slope of approximated curve 0.30 0.29 0.29 0.290.31 0.35 Core coefficient of restitution 0.015 0.004 0.017 0.021 0.0130.018 Core compression deformation amount (mm) 3.75 3.80 3.27 3.34 3.843.81 Peak intensity (1560 ± 30 cm⁻¹) of difference 0.042 0.057 0.0780.074 0.039 0.044 spectrum between the center and surface of the coreCover composition A A A A A A Cover hardness (Shore D) 65 65 65 65 65 65Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm)−90 −50 −80 −90 −100 −120 Driver flying distance (m) 4.0 2.0 2.4 3.2 3.24.4 Coefficient of restitution 0.015 0.004 0.017 0.021 0.013 0.018Compression deformation amount (mm) 3.05 3.10 2.57 2.64 3.14 3.11

TABLE 6 Golf ball No. 21 22 23 24 25 Rubber composition BR730 100 100100 100 100 (parts by mass) Sanceler SR 29 29 29 29 29 Zinc oxide 5 5 55 5 Barium sulfate *1) *1) *1) *1) *1) 2-Thionaphthol — — 0.1 — —2,4,5-TCTP 0.42 — — 0.42 — 2,6-DCTP — 0.11 — — 0.11 Zinc stearate — — —— — Zinc myristate — — — — — Zinc octanoate 5 5 — — — Dicumyl peroxide0.8 0.8 0.8 0.8 0.8 Total amount of carboxylic acid/salt 7.9 7.9 2.9 2.92.9 Core hardness distribution Center hardness 46.8 47.5 53.7 52.8 53.4(JIS-C) 12.5% point hardness 52.9 55.6 63.4 64.4 64.8 25% point hardness59.8 63.1 69.1 68.7 69.4 37.5% point hardness 63.5 67.0 70.4 69.9 70.750% point hardness 65.0 68.8 70.5 70.1 70.7 62.5% point hardness 67.069.9 69.7 69.4 69.7 75% point hardness 74.0 77.0 72.5 72.2 73.3 87.5%point hardness 76.4 79.7 78.0 77.4 78.2 Surface hardness 83.8 85.3 84.985.1 85.0 Surface hardness − center hardness 37.0 37.8 31.2 32.3 31.6 R²of approximated curve 0.97 0.96 0.84 0.82 0.83 Slope of approximatedcurve 0.33 0.34 0.23 0.23 0.23 Core coefficient of restitution 0.0040.015 0.008 0.016 0.022 Core compression deformation amount (mm) 4.203.58 3.66 3.89 3.61 Peak intensity (1560 ± 30 cm⁻¹) of difference 0.0800.040 0.018 0.012 0.010 spectrum between the center and surface of thecore Cover composition A A A A A Cover hardness (Shore D) 65 65 65 65 65Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm)−110 −100 0 0 0 Driver flying distance (m) 4.2 4.2 0 0 0 Coefficient ofrestitution 0.004 0.015 0.008 0.016 0.022 Compression deformation amount(mm) 3.50 2.88 2.96 3.19 2.91

TABLE 7 Golf ball No. 26 27 28 29 Rubber composition BR730 100 100 100100 (parts by mass) Sanceler SR 27 29 29 29 Zinc oxide 5 5 5 5 Bariumsulfate *1) *1) *1) *1) 2-Thionaphthol 0.32 0.32 0.32 0.32 Aluminumstearate 10 — — — Magnesium stearate — 5 — — Calcium stearate — — 5.5 —Cobalt stearate — — — 11 Dicumyl peroxide 0.8 0.8 0.8 0.8 Total amountof carboxylic acid/salt 12.7 7.9 8.4 13.9 Core hardness Center hardness56.2 52.8 49.1 51.3 distribution (JIS-C) 12.5% point hardness 60.6 57.055.8 55.6 25% point hardness 63.7 60.5 60.7 60.0 37.5% point hardness65.2 61.7 62.6 61.7 50% point hardness 65.5 62.5 63.4 62.5 62.5% pointhardness 69.4 67.6 66.0 65.4 75% point hardness 76.4 74.9 71.2 72.387.5% point hardness 77.6 74.1 75.4 76.3 Surface hardness 83.0 79.7 80.480.7 Surface hardness − center hardness 26.8 26.9 31.3 29.4 R² ofapproximated curve 0.96 0.96 0.97 0.97 Slope of approximated curve 0.250.26 0.28 0.28 Core coefficient of restitution 0.012 −0.004 −0.005−0.007 Core compression deformation amount (mm) 4.11 4.42 4.52 4.56 Peakintensity (1560 ± 30 cm⁻¹) of difference 0.058 0.040 0.028 0.047spectrum between the center and the surface of the core Covercomposition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness(mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −40 −40 −60 −60 Driverflying distance (m) 1.7 1.6 2.0 2.0 Coefficient of restitution 0.012−0.004 −0.005 −0.007 Compression deformation amount 3.41 3.72 3.82 3.86(mm) *1) In tables No. 3 to 7, as to an amount of barium sulfate,adjustment was made such that the golf ball had a mass of 45.4 g.BR730: a high-cis polybutadiene (cis-1,4 bond content=96 mass 1,2-vinylbond content=1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.)=55, molecularweight distribution (Mw/Mn)=3) available from JSR CorporationSanceler SR: zinc acrylate (product of 10 mass % stearic acid coating)available from Sanshin Chemical Industry Co., Ltd.Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd.Barium sulfate: “Barium sulfate BD” manufactured by Sakai ChemicalIndustry Co., Ltd., adjustment was made such that the finally obtainedgolf ball had a mass of 45.4 g.2-thionaphthol: available from Tokyo Chemical Industry Co., Ltd.2,4,5-TCTP: 2,4,5-trichlorothiophenol available from Tokyo ChemicalIndustry Co., Ltd.2,6-DCTP: 2,6-dichlorothiophenol available from Tokyo Chemical IndustryCo., Ltd.Dicumyl peroxide: “PERCUMYL® D” available from NOF Corporation.Stearic acid: available from Tokyo Chemical Industry Co., Ltd., (purityof 98% or higher).Zinc octanoate: available from Mitsuwa Chemicals Co., Ltd. (purity of99% or higher).Zinc myristate: available from NOF Corporation (purity of 90% orhigher).Zinc stearate: available from Wako Pure Chemical Industries (purity of99% or higher).Aluminum stearate: available from Mitsuwa Chemicals Co., Ltd.Magnesium stearate: available from Wako Pure Chemical Industries.Calcium stearate: Tokyo Chemical Industry Co., Ltd.Cobalt stearate: CO-ST-F available from DIC Corporation.(2) Production of Cover

Cover materials shown in Table 8 were mixed with a twin-screw kneadingextruder to prepare the cover compositions in the pellet form. Theextruding conditions of the cover composition were a screw diameter of45 mm, a screw rotational speed of 200 rpm, and, screw L/D=35, and themixtures were heated to 150 to 230° C. at the die position of theextruder. The cover compositions obtained above were injection-moldedonto the spherical cores to produce the golf balls having the sphericalcore and the cover covering the spherical core. Results of evaluatingthe properties of the obtained golf balls were also shown in Tables 3 to7.

TABLE 8 Cover composition A B Himilan 1605 50 — Himilan 1706 50 —Elastollan NY97A — 100 Titanium oxide  4 4 Slab hardness (Shore D) 65 47Formulation: parts by mass Himilan 1605: Sodium ion neutralizedethylene-methacrylic acid copolymer ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd Himilan 1706: Zinc ion neutralizedethylene-methacrylic acid copolymer ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd Elastollan NY97A: Thermoplasticpolyurethane elastomer available from BASF Japan Co.

The results of Tables 3 to 7 indicate that the golf balls comprising aspherical core and at least one cover layer covering the spherical core,wherein the spherical core has an absorbance difference betweenabsorbance measured at a surface thereof and absorbance measured at acenter thereof of 0.024 or more, with respect to an absorption peakappeared at 1560±30 cm⁻¹ when measuring the spherical core with aFourier transform infrared spectrophotometer, travel a great flightdistance.

FIG. 24 shows the result of the FT-IR measurement of the spherical core(spherical body No. 3) molded from the rubber composition where myristicacid is added as (d) the carboxylic acid in an amount of 10 parts bymass with respect to 100 parts by mass of (a) the base rubber. FIG. 24(a) shows the result of the measurement at the spherical core center.FIG. 24 (b) shows the result of the measurement at the spherical coresurface, and FIG. 24 (c) shows a difference spectrum. The absorbancedifference of the absorption peak appeared at 1538 cm⁻¹ was 0.065.

In table 9, spherical cores were taken from the commercial golf balls Ato G, and the results of measuring surfaces and centers of the sphericalcores with a Fourier transform infrared spectrophotometer were shown.Since the commercial products F and C have two-layered cores, and theabsorbance differences between the surface and the center of the innercore were shown. With respect to the absorption peak appeared at 1560±30cm⁻¹, the absorbance differences between absorbance measured at asurface thereof and absorbance measured at a center thereof were lessthan 0.024, respectively.

TABLE 9 Absorbance difference between the surface and center Golf ballof the core at 1540 cm⁻¹ Remark Commercial product A 0.011 — Commercialproduct B 0.009 — Commercial product C 0.010 — Commercial product D0.010 — Commercial product E 0.004 — Commercial product F 0.018Two-layered core Commercial product G 0.023 Two-layered core

The golf ball of the present invention travels a great flight distanceon driver shots. This application is based on Japanese Patentapplication No. 2011-183084 filed on Aug. 24, 2011.

The invention claimed is:
 1. A golf ball having a spherical core and atleast one cover layer covering the spherical core, wherein the sphericalcore is formed from a rubber composition comprising: (a) a base rubber,(b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/ora metal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, (d) a carboxylic acid having 1 to 30 carbon atoms and/or asalt thereof in a content ranging from 0.1 part by mass to 40.0 parts bymass with respect to 100 parts by mass of the base rubber, saidcarboxylic acid excluding an α,β-unsaturated carboxylic acid having 3 to8 carbon atoms or a metal salt thereof, and (f) an organic sulfurcompound in a content ranging from 0.05 parts to 5 parts by mass withrespect to 100 parts by mass of the base rubber, provided that therubber composition further contains (e) a metal compound if only anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is present asthe co-crosslinking agent, wherein the spherical core exhibits adifference in infrared absorbance measured at a surface thereof andmeasured at a center thereof of 0.024 or more, with respect to anabsorption peak appearing at 1560±30 cm⁻¹ when measuring the sphericalcore with a Fourier transform infrared spectrophotometer, the sphericalcore has a hardness difference ranging from 18 to 60 in JIS-C hardnessbetween a surface hardness and a center hardness thereof, and if JIS-Chardness values are measured at nine points obtained by dividing aradius of the spherical core into equal parts having 12.5% intervalstherebetween so as to include the center and the surface, and thehardness values are plotted on a graph against distance (%) includingthe core center and core surface, then R² of a linear approximationcurve obtained from the least square method is 0.95 or higher.
 2. Thegolf ball according to claim 1, wherein the spherical core is asingle-layered core.
 3. The golf ball according to claim 1, wherein (d)the carboxylic acid and/or the salt thereof is a fatty acid and/or asalt thereof.
 4. The golf ball according to claim 1, wherein (d) thecarboxylic acid and/or the salt thereof is a carboxylic acid having 1 to14 carbon atoms, and the rubber composition contains (d) the carboxylicacid in a content ranging from 0.1 part by mass to 13.0 parts by masswith respect to 100 parts by mass of (a) the base rubber.
 5. The golfball according to claim 1, wherein (d) the carboxylic acid and/or thesalt thereof is a salt of a carboxylic acid having 1 to 14 carbon atoms,and the rubber composition contains (d) the salt of the carboxylic acidin a content ranging from 0.1 part by mass to 30.0 parts by mass withrespect to 100 parts by mass of (a) the base rubber.
 6. The golf ballaccording to claim 1, wherein (f) the organic sulfur compound includesthiophenol or a derivative thereof, diphenyl disulfide or a derivativethereof, thionaphthol or a derivative thereof, thiuram disulfide or aderivative thereof, or a metal salt of these compounds.
 7. The golf ballaccording to claim 1, wherein the rubber composition contains (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof in a content ranging from 15 parts to 50 parts bymass with respect to 100 parts by mass of (a) the base rubber.
 8. Thegolf ball according to claim 4, wherein (d) the carboxylic acid and/orthe salt thereof is a carboxylic acid having 1 to 9 carbon atoms.
 9. Thegolf ball according to claim 5, wherein (d) the carboxylic acid and/orthe salt thereof is a salt of a carboxylic acid having 1 to 9 carbonatoms.
 10. The golf ball according to claim 1, wherein the carboxylicacid (d) has 5 to 30 carbon atoms and the α,β-unsaturated carboxylicacid or the metal salt thereof (b) has 3 to 4 carbon atoms.
 11. The golfball according to claim 1, wherein the carboxylic acid (d) is analiphatic carboxylic acid.
 12. The golf ball according to claim 10,wherein the carboxylic acid (d) is an aliphatic carboxylic acid.
 13. Thegolf ball according to claim 1, wherein the spherical core has a centerhardness of 54.4 or less in JIS-C hardness.
 14. The golf ball accordingto claim 1, wherein the spherical core has a center hardness of 51.6 orless in JIS-C hardness.